The mechanisms directing cardiac hypertrophy, triggered by hemodynamic overload and/or structural heart disease, are still not completely understood. Delineating how cardiac myocytes transduce mechanical stress related signaling from the membrane to nuclear transcription will greatly contribute to our understanding of the clinical problems of cardiac hypertrophy. RhoA, a small molecular weight GTP- binding protein, acts as a molecular switch that controls various cell functions, and is a potential mediator of hypertrophic signals. We hypothesize that RhoA functions as a key transducer in the induction cascade that links stretch dependent signaling to the induction of cardiac gene expression in vivo. In addition, we propose that the Ser/Thr kinase P160rock, recently identified as a downstream effector of Rho1A, is the primary signaling molecule mediating hypertrophic effects of RhoA. Our preliminary results indicate that RhoA and p160ROCK, together with beta1 integrin and its adhesion dependent actions, serve as a novel signal pathway in activating the cardiac enriched serum response factor (SRF) dependent genes in cardiomyocytes, such as skeletal alpha-actin, through the p160ROCK dependent phosphorylation of SRF. We have also observed that the level of p160ROCK mRNA transcripts is markedly induced in the overloaded heart, suggesting an important role for this molecule in the development of cardiac hypertrophy. Our results suggest that phosphorylation of SRF by 160ROCK might be a critical step for the activation of fetal cardiac gene expression during cardiac hypertrophy. The Specific Aims of this proposal are: 1) to determine the role of RhoA, p160ROCK and SRF in mediating hypertrophic responses induced by stretch in cultured cardiomyocytes; 2) to determine if the RhoA signaling pathways regulates SRF-dependent hypertrophic gene expression via phosphorylation of SRF by 160ROCK; 3) to determine the role of RhoA and p160ROCK in mediating cardiac hypertrophic responses induced by pressure overload in genetically manipulated mouse models. The overall objective is to determine if RhoA, beta1 integrin, p160ROCK and SRF are primary mediators of a potentially important novel signal pathway that contributes to reprogramming of cardiomyocyte gene expression in stretch induced cardiac hypertrophy.